FTDs (e.g., mixers, converters, and tuners) are devices that convert radio frequencies (RF) to intermediate frequencies (IF) when used for down-conversion or IFs to RFs when used for up-conversion. In a three-port mixer, the down-conversion of an RF to an IF or the up-conversion of an IF to an RF involves mixing an input signal with a local oscillator (LO) signal. Mixing an input signal with an LO signal generates two primary output signals having frequencies that are the sum of the input signal frequency and the LO signal frequency and the difference between the input signal frequency and the LO signal frequency. In down-conversion, an RF signal is mixed with an LO signal to generate a lower frequency IF signal and in up-conversion an IF signal is mixed with an LO signal to generate a higher frequency RF signal.
As the demand for wireless communications services increases, more advanced FTDs with tighter operating tolerances are being developed. To support the development of new FTDs and the integration of FTDs into RF communications systems, the conversion responses of FTDs need to be characterized. Two conversion responses of FTDs that are typically characterized include conversion loss and phase shift. The conversion loss of an FTD is a measure of the difference in amplitude between the input signal (the RF in down-conversion and the IF in up-conversion) and the output signal (the IF in down-conversion and the RF in up-conversion). The phase shift of an FTD is a measure of the phase shift between the input signal and the output signal.
A technique for characterizing the conversion response of an FTD is disclosed in U.S. Pat. No. 5,937,006 issued to Clark et al. (hereinafter Clark). Clark discloses a measurement technique for characterizing an FTD that involves making at least three different measurements using three different pairs of FTDs, where the three different pairs of FTDs are put together from a set of three different FTDs. According to the three-pair measurement technique, one of the FTDs is the device under test (DUT) and the other two FTDs are test devices (referred to as test mixer 1, TM1, and test mixer 2, TM2) that support the characterization of the DUT. FIG. 1 depicts the basic structure of a three-pair measurement system 100 for characterizing the single side band conversion response of an FTD. The three-pair measurement system includes a vector network analyzer (VNA) 102, a controller 104, an up-conversion FTD 110, a down-conversion FTD 112, connections 106 and 108 between the VNA and the up-conversion and down-conversion FTDs, an optional phase shifter 114, an LO 116, and a splitter 118. Although FIG. 1 depicts the basic structure of a three-pair measurement system, an actual implementation may include additional components, such as filters, attenuators, and isolators, which are used to improve the measurement quality. Characterizing the single side band response of a DUT FTD using the three-pair measurement system involves three separate measurements with the DUT, TM1, and TM2 used in different combinations as the up-conversion FTD and the down-conversion FTD. The connections 106 and 108 couple the FTDs to the VNA and may include ports and connection cables as is known in the field.
FIG. 2 is a measurement flow diagram for characterizing the conversion response of a single side band DUT. The flow diagram indicates the different combinations of the DUT, TM1, and TM2 that are used to characterize the DUT. In measurement A (step 220), an IF is input into the up-conversion FTD (the DUT) for up-conversion to an RF and then into the down-conversion FTD (TM1) for down-conversion back to an IF. The result of measurement A is a measurement MA(f) that is used to determine a response RA(f). In measurement B (step 222), an IF is input into the up-conversion FTD (the DUT) for up-conversion to an RF and then into the down-conversion FTD (TM2) for down-conversion back to an IF. The result of measurement B is a measurement MB(f) that is used to determine a response RB(f). In measurement C (step 224), an IF is input into the up-conversion FTD (TM1) for up-conversion to an RF and then into the down-conversion FTD (TM2) for down-conversion back to an IF. The result of measurement C is a measurement MC(f) that is used to determine a response RC(f). The single side band response of the DUT is then calculated (step 226) as RDUT(f)=[RA(f)+RB(f)−RC(f)]/2.
One requirement of the three-pair measurement system is that one of the test FTDs (either TM1 or TM2) must have a reciprocal conversion response. That is, the conversion response of the reciprocal FTD must be the same whether the FTD is used as a down-conversion FTD 112 or as an up-conversion FTD 110. In the three-pair test system, the reciprocal FTD is the FTD that is used as the down-conversion FTD in one of the measurements and as the up-conversion FTD in another one of the measurements. Referring to the flow diagram of FIG. 2, TM1 is used as the down-conversion FTD in measurement A (step 220) and as the up-conversion FTD in measurement C (step 224). To accurately determine the conversion response of the DUT, TM1 must have a reciprocal conversion response.
Referring back to FIG. 1, the basic structure of a three-pair measurement system 100 for characterizing the double side band conversion response of a DUT is the same as the measurement system for characterizing a single side band response except that the measurement system includes the phase shifter 114. In the embodiment of FIG. 1, the phase shifter is located between the LO 116 and the down-conversion FTD 112 so that the phase of the LO can be shifted before it is inserted into the down-conversion FTD.
FIG. 3 is a measurement flow diagram for characterizing a double side band response of a DUT. The flow diagram indicates the different combinations of the DUT, TM1, TM2, and the phase of the LO that are used to characterize the DUT. The double side band characterization involves initial measurements A, B, and C (steps 320, 322, and 324) that are similar to the A, B, and C measurements (steps 220, 222, and 224) described above with reference to FIG. 2. The double side band characterization includes additional measurements A′, B′, and C′ that are made with the same FTD configuration however the phase of the LO that is inserted into the down-conversion FTD is shifted by 90 degrees to obtain the A′, B′, and C′ measurements. This technique enables calculation of the upper side band (USB) and the lower side band (LSB) conversion responses of the DUT. As shown in FIG. 3 at step 326, the USB conversion response of the DUT is calculated as RDUT USB(f)=[RA USB(f)+RB USB(f)−RC USB(f)]/2 and the LSB conversion response of the DUT is calculated as RDUT LSB(f)=[RA LSB(f)+RB LSB(f)−RC LSB(f)]/2.
As with the single side band measurement technique, one of the test FTDs (either TM1 or TM2) must have a reciprocal conversion response. Referring to FIG. 3, TM1 is used as the down-conversion FTD in the A and A′ measurements (step 320) and is used as the up-conversion FTD in the C and C′ measurements (step 324). To determine the conversion response of the DUT, TM1 must have a reciprocal conversion response.
FIGS. 4, 5, and 6 depict examples of well-known three-port mixer FTDs that can be utilized as the test mixers in a three-pair measurement system. The mixers are passive diode-based mixers in which the mixer diode(s) 430, 530, and 630 is/are driven by the LO of the test system. The mixer of FIG. 4 is an example of a single diode mixer, the mixer of FIG. 5 is an example of a single balanced mixer, and the mixer of FIG. 6 is an example of a double balanced mixer (DBM). Typical solid state mixer diodes have a threshold voltage of 0.3 volts and are biased by an LO. FIG. 7 depicts the voltage modulation of an example LO, having an amplitude of 0.3 volts, that can be used to bias the mixers of FIGS. 4–6. The voltage modulation of the passive diode-based mixers causes the mixer diodes to be forward biased and reversed biased in an alternating fashion, thereby mixing an RF with the LO to generate an IF or mixing an IF with the LO to generate an RF.
Although the three-pair measurement system for characterizing an FTD requires at least one reciprocal FTD to provide quality measurement results, current passive diode-based mixers, such as the example mixers depicted in FIGS. 4–6, do not exhibit the required reciprocal conversion response. Typically, passive diode-based mixers exhibit greater conversion loss during down-conversion than during up-conversion. Clark discloses a calibration technique that involves adjusting multiple attenuators to reduce reflections and optimize conversion linearity as a way to ensure that at least one of the FTDs has a reciprocal conversion response. Many measurements of mixers show that linearity alone is insufficient to guarantee reciprocity, and reflections are irrelevant with respect to reciprocity.
In view of the need to characterize FTDs, especially three-port mixer FTDs and in view of the need to utilize an FTD with a reciprocal conversion response in a s three-pair measurement system and method, what is needed is a reciprocal FTD that can be incorporated into a three-pair measurement system and method.